The present invention relates to hardening gears, particularly but not exclusively of large welded gears.
Gears to be hardened in accordance with the invention are, for example, composed of two annular disks having additional perforations and being welded onto a hub. The gear proper is established by a rim element to which the peripheries of the disks are welded. However, this special kind of gear is referred to here only because the invention arose from particular problems discovered in the current methods of hardening such gears. The inventive method may well have broad application in the art of quench-hardening other types of gears.
Gears of the type referred to above have been hardened by heating them in a carbon atmosphere to obtain carburization and by subsequently quenching the gear in a saline bath. It was discovered that the stengthening of gears, particularly for increasing the load bearing capability of the surfaces of the teeth, was limited by this quenching method which is usually followed by grinding.
Gears are made today which have a diameter up to 2700 mm (a little more than 10") and a gear teeth width (axial length) of 700 mm (a little less than 3"). It was found that hardening such gear is inadequate by, e.g. exposing the gear to a carbon atmosphere at a temperature between 900.degree. or 950.degree. C. in a pit furnace to obtain carburization and to quench the gears subsequently in a saline bath at 150.degree. to 190.degree. C.; the gears having a reduced temperature of about 820.degree. to 850.degree. just prior to dipping into the quenching bath.
We discovered that a major problem arises from the fact that the entire wheel is carburized, particularly, also, the welding seams, web elements and disks. Upon quenching, the disks cool faster than hub and rim so that the gear as a whole undergoes deformations (in addition to irregularities in the shape due to to tolerances, etc.). This resulting distortion, in turn, requires a more than desired amount of teeth grinding and polishing.
It was further observed that one of the main contributing factors to the irregular quenching relates to the flow of saline liquid in the bath. As long as the quenching vessel is empty, a rather uniform flow of the saline liquid of the bath can be established. As soon as a gear, particularly a large gear, is placed in the bath, the flow pattern becomes highly irregular, not only by the gear itself but also by the device dipping the gear into the bath and holding it therein. To equalize the flow or, better, to control the flow pattern under these conditions requires additional liquid circulating equipment which, moreover, has to be changed or, at least, readjusted or repositioned for gears of different sizes and dimensions, because each instance presents a different flow condition.